Piezoelectricity in cellular electret films

Permanently charged films with a cellular or porous structure represent a n ew family of polymer electrets. These materials show piezoelectric properti es with high piezoelectric constants. The electromechanical response equati ons of such films are derived for their operation as sensors and as actuato rs. Experimental results are also presented for cellular polypropylene (PP) . In particular, measurements of the direct and inverse piezoelectric const ants in the frequency range 0 to 10 kHz and of the variation of these const ants across the surface of the films are discussed. These measurements, per formed by direct application of stress or by the use of a profilometer, an accelerometer and an interferometer yield a frequency-independent piezoelec tric d(33) constant of less than or similar to 220 pC/N. Assuming reasonabl e charge distributions and charge densities, the calculated piezoelectric c onstants are in good agreement with the measured values. The theoretical mo del shows the reciprocity of the piezoelectric constants.